The present invention relates to power supplies and in particular to power factor correction (PFC) power supplies.
Electric power is distributed almost universally in an alternating current (AC) format that allows for efficient transmission. Most devices however, including personal computers, televisions, etc., require direct current (DC) power. Power supplies (or converters) act to convert the AC input supplied by a line to a DC output suitable for consumption by a device or load, or act to convert a DC input to a DC output (i.e., a DC-to-DC converter). A switched-mode power supply (SMPS) employing a boost regulator is commonly employed in this role of AC-to-DC or DC-to-DC power conversion. A benefit of employing a SMPS having a boost regulator topology is the boost regulator can be controlled to provide power factor correction, wherein the term “power factor correction” refers to the efficiency of the circuit (i.e., real power versus apparent power). Power factor correction typically involves ensuring that the current drawn by the PFC circuit is in-phase with the voltage provided by the PFC boost regulator. Subsequent stages may be employed to step-down the output of the PFC boost regulator to a desired DC output voltage, and other topologies may be employed in conjunction with or instead of boost regulator topologies, such as flyback topologies.
A distinction between PFC power supplies can be made between those power supplies that include a first stage rectifier bridge and so-called bridgeless PFC boost regulators. The first stage rectifier bridge includes four diodes, connected in a bridge topology to rectify the AC input supplied by the line. The voltage drops associated with each diode represent losses that lower the overall efficiency of the power supply. Improvements to PFC power supplies have included the use of half-bridge designs that employ two diodes instead of four, and bridgeless PFC power supplies that further reduce the number of semiconductor devices (e.g., diodes, switches) required in any given circuit path, thereby further improving efficiency. However, typical bridgeless PFC power supply topologies introduce common-mode electromagnetic interference (EMI) that is difficult to filter.